Choline is necessary for the normal function of mammalian organisms. It is a precursor for the biosynthesis of phospholipids - essential components of all membranes and it is needed to make acetylcholine, a neurotransmitter. Newborn rats and humans probably require especially large amounts of choline to sustain rapid growth. The diet is a major source of choline and choline-containing phospholipids. We know little about the mechanisms responsible for the absorption of choline and choline-containing compounds from gut lumen. In the neonate these mechanisms have never been studied. We propose experiments which will characterize the absorption of these compounds from the gut of 10-day-old, 21-day-old and adult rats. Choline absorptions can be divided into 4 phases - a lumenal component (where choline remains within the lumenal space and is subjected to enzymatic and bacterial actions), a mucosal component (which consists of bidirectional transfer of choline from lumen to enterocyte), a capillary component (which consists of bidirectional transfer of choline from within the enterocyte to the blood), and a paracellular component (which consists of choline transfer across the tight junction into the intercellular space). We will use gut slices, vesicles made from enterocyte brush border and basolateral membranes, in situ perfused cannulated loops of gut, and whole rats to study each of these phases of choline transport. Mechanisms within the duodenum, jejunum, ileum and colon will be characterized. Studies which will characterize phosphatidylcholine (lecithin) and sphingomyelin transport by gut will be initiated.